UV-B-induced differential transcription of psbA genes encoding the D1 protein of photosystem II in the Cyanobacterium synechocystis 6803.

UV-B irradiation of intact Synechocystis sp. PCC 6803 cells results in the loss of photosystem II activity, which can be repaired via de novo synthesis of the D1 (and D2) reaction center subunits. In this study, we investigated the effect of UV-B irradiation on the transcription of the psbA2 and psbA3 genes encoding identical D1 proteins. We show that UV-B irradiation increases the level of psbA2 mRNA 2-3-fold and, more dramatically, it induces a 20-30-fold increase in the accumulation of the psbA3 mRNA even at levels of irradiation too low to produce losses of either photosystem II activity or D1 protein. The induction of psbA3 transcript accumulation is specific for UV-B light (290-330 nm). Low intensity UV-A emission (330-390 nm) and white light induce only a small, at most, 2-3-fold enhancement, whereas no effect of blue light was observed. Expression patterns of chimeric genes containing the promoter regions of the psbA2, psbA3 genes fused to the firefly luciferase (luc) reporter gene indicate that (i) transcription of psbA2/luc and psbA3/luc transgenes was elevated, similarly to that of the endogenous psbA genes, by UV-B irradiation, and that (ii) a short, 80-base pair psbA3 promoter fragment is sufficient to maintain UV-B-induced transcription of the luc reporter gene. Furthermore, our findings indicate that UV-B-induced expression of the psbA2 and psbA3 genes is a defense response against UV-B stress, which is regulated, at least, partially at the level of transcription and does not require active electron transport.